123 lines
5.8 KiB
HTML
123 lines
5.8 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2016 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>GNU Compiler Collection (GCC) Internals: Portability</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Portability">
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<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Portability">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="index.html#Top" rel="up" title="Top">
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<link href="Interface.html#Interface" rel="next" title="Interface">
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<link href="Contributing.html#Contributing" rel="prev" title="Contributing">
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<style type="text/css">
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-->
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Portability"></a>
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<div class="header">
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<p>
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Next: <a href="Interface.html#Interface" accesskey="n" rel="next">Interface</a>, Previous: <a href="Contributing.html#Contributing" accesskey="p" rel="prev">Contributing</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="GCC-and-Portability"></a>
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<h2 class="chapter">2 GCC and Portability</h2>
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<a name="index-portability"></a>
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<a name="index-GCC-and-portability"></a>
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<p>GCC itself aims to be portable to any machine where <code>int</code> is at least
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a 32-bit type. It aims to target machines with a flat (non-segmented) byte
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addressed data address space (the code address space can be separate).
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Target ABIs may have 8, 16, 32 or 64-bit <code>int</code> type. <code>char</code>
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can be wider than 8 bits.
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</p>
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<p>GCC gets most of the information about the target machine from a machine
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description which gives an algebraic formula for each of the machine’s
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instructions. This is a very clean way to describe the target. But when
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the compiler needs information that is difficult to express in this
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fashion, ad-hoc parameters have been defined for machine descriptions.
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The purpose of portability is to reduce the total work needed on the
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compiler; it was not of interest for its own sake.
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</p>
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<a name="index-endianness"></a>
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<a name="index-autoincrement-addressing_002c-availability"></a>
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<a name="index-abort"></a>
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<p>GCC does not contain machine dependent code, but it does contain code
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that depends on machine parameters such as endianness (whether the most
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significant byte has the highest or lowest address of the bytes in a word)
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and the availability of autoincrement addressing. In the RTL-generation
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pass, it is often necessary to have multiple strategies for generating code
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for a particular kind of syntax tree, strategies that are usable for different
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combinations of parameters. Often, not all possible cases have been
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addressed, but only the common ones or only the ones that have been
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encountered. As a result, a new target may require additional
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strategies. You will know
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if this happens because the compiler will call <code>abort</code>. Fortunately,
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the new strategies can be added in a machine-independent fashion, and will
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affect only the target machines that need them.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Interface.html#Interface" accesskey="n" rel="next">Interface</a>, Previous: <a href="Contributing.html#Contributing" accesskey="p" rel="prev">Contributing</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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